The ease with which a tire becomes unseated, which means to say with which the bead of the tire moves around on, and then comes off the rim, which means to say becomes detached from the rim when a transverse load is applied to the tire, is one of the tire features important to the safety of the passengers of the vehicle.
Unseating is more particularly a problem for the following low-pressure tires:                tires for sporty vehicles where there is a desire to improve grip by having a contact patch that is increased by reducing the service pressure,        tires for agricultural vehicles where there is a desire to reduce the impact on the compaction of cultivated land by reducing the service pressure,        passenger-vehicle tires which are designed to run for a few hundred kilometres following a loss of pressure.        
For example, for passenger-vehicles, the standardization authorities have, with respect to unseating and tires coming off the rim, set target values to be achieved (such as, for example, in Chinese standard GB/T 2978-2008) and corresponding test methods (such as, for example, the method recommended in Chinese standard GB/T 4502-2009). An example of a test method is also described in patent U.S. Pat. No. 3,662,597, whereby a conical form is applied against a sidewall of the tire. The pressure applied to the sidewall is then increased and the sidewall movement is recorded. A similar test method has been adopted in “Federal Motor Vehicle Safety Standard No. 109” by the American federal road safety organisation the NHTSA (National Highway Traffic Safety Administration).
These increasingly demanding recommendations from the standardization authorities require these tires to be made even more resistant to unseating and to coming off their rims.
Conventionally, the resistance to unseating can be increased by bonding the tire to the mounting rim, although this solution has the disadvantage of making the tire very difficult to remove. It is also possible to modify the geometry of the tire and of the rim, as disclosed in documents EP103346 B1 or EP1307351 B1 or EP358490 A2, so as to make unseating more difficult, but this approach entails destandardizing the rim and sometimes expensive modifications to the moulds used to manufacture the tires.
Since a tire has a geometry exhibiting symmetry of revolution with respect to an axis of rotation, the geometry of the tire was generally described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial, axial and circumferential directions respectively denote the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire and perpendicular to the meridian plane.
In what follows, the expressions “radially on the inside of” and “radially on the outside of” respectively mean “closer to the axis of rotation of the tire, in the radial direction, than” and “further from the axis of rotation of the tire, in the radial direction, than”. The expressions “axially on the inside of” and “axially on the outside of” respectively mean “closer to the equatorial plane, in the axial direction, than” and “further away from the equatorial plane, in the axial direction, than”. A “radial distance” is a distance with respect to the axis of rotation of the tire and an “axial distance” is a distance with respect to the equatorial plane of the tire. A “radial thickness” is measured in the radial direction, and an “axial width” is measured in the axial direction.
The expression “rubber compound” denotes a composition of rubber containing at least an elastomer and a filler.